I'm trying to figure out exactly what you're wanting. If I'm correct on this, you're using a regular amp with the HXS in front, using the HXS for preamp modeling, but using the amplifier power amp and cabinet?
When you say amp in is that the instrument input or the effects loop of the amp?
I'm not sure if this would work or not, but the HXFX has 2 effects loops. You could run the modulation effects (chorus, flanger, etc.) in front of the preamp on the HXS, create an effects loop in the HXFX, and connect it via 4CM to the HXS by creating an effects loop in the HXS. This way your delays, reverbs, etc. will be after the preamp of the HXS, but before the power amp of your amplifier.
So signal path would be: Guitar -> HXFX mono in -> HXFX mono out -> HXS mono in -> HXS Mono out -> amplifier in.
I I
HXFX Send (1 or 2) -------------------------HXS Return
Again, I don't know if this would work or not.